i think this video might be useful, it is quite hard to explain, hope it helps! https://www.youtube.com/watch?v=Klnroe1UBRs

Here is a page of phrases with 'whether' and their Spanish equivalents: http://dictionary.reverso.net/english-spanish/whether I hope it'll help you.

Whether is like a choice when comparing something. It is usually "whether" to do one thing or the other, to choose "this" or "that" To use your example, whether at twenty or eighty would explicitly mean "to choose to stop learning at twenty means you will grow old OR if you choose to stop learning at eighty you will grow old. My example would be; "I am trying to decide whether to go to the beach or the park today". Whether in this case is used to show or express that I am comparing the two options in order to make a choice. Which should I do? I will go to the beach. Hope this helps, let me know if you need more clarification. Good luck

You usually use is when you are making a choice or doubt between two situations. e.g. Whether we should do it or not or Whether you take the an offer or decline it.

Whether = "regardless" .. or "in spite of"